Conventionally, a step unit is provided on a vehicle main body to be adjacent to a vehicle sliding door thereof. For example, referring to Non-Patent Document 1, a step unit includes a step member (step) and a rail plate member. A lower rail extending in the opening-closing direction of a vehicle sliding door is provided on the lower surface of the step member. The lower rail supports rollers coupled to the sliding door, so that the rollers and the sliding door are guided along the lower rail. Such a step unit has a cutout portion formed in a part of the lower rail. With the rail plate member (sliding door lower rail plate) removed, the cutout portion allows rollers to be supported by the lower rail or to be removed from the lower rail.